Polymer beads of substantially uniform size and shape are required for various industrial applications such as the production of ion-exchange resins, or manufacturing processes involving molding or extrusion.
However, known processes which comprise polymerizing monomer droplets in suspension generally provide non-uniform polymer beads in a relatively broad size range, for example from about 0.1 mm to about 1.3 mm.
Subsequent screening steps are thus necessary in order to provide beads in several more restricted size ranges, which entails significant screening and storage costs, as well as the rejection of commercially unusable beads produced.
Uniform droplets may be produced by various known devices comprising for example calibrated tubes or vibrating nozzles which must be adapted to the droplet size required in each case, and are not particularly suitable for industrial manufacturing processes.
An investigation of the phenomenon of "limited coalescence" of oil droplets in water is discussed in an article by R. M. Wiley, published in the Journal of Colloid Science, Vol. 9, No 5, Oct. 1954, p. 427-437, and incorporated by reference herein. This article shows more particularly that a colloid or finely divided, dispersable solid agent, which will be called hereinafter a surface protective agent, is essential for coalescence up to a limiting droplet size which is directly proportional to the product of the dispersed (coalescing) phase volume and the solid particle size (colloid), and inversely proportional to the weight of solid colloid dispersed in the water.
It is nevertheless particularly difficult to produce uniform polymer beads since the monomer droplets in suspension should be kept as free as possible from collisions while they are slowly solidified in the course of polymerization. On the other hand, if the monomer droplets are suspended in an agitated liquid polymerizing medium, monomer droplets which undergo collisions while they are still liquid or in an intermediate gelled state may either break up into smaller droplets, or stick together to form larger droplets. U.S. Pat. No. 2,934,530 may be cited in this connection.
In addition, French Pat. Nos. 1,485,547 and 1,469,922, German Democratic Republic Pat. Nos. 99386 and 61099, German Federal Republic published patent application DE-OS 2,402,674 may be cited to further illustrate the state of the art relating to suspension polymerization.
As may be seen from the above explanations and from the cited prior art, the continuous production of polymer beads of controlled size is particularly difficult to achieve on an industrial scale.